(1) Field of the Invention
The present invention relates to an easily openable vessel closure and a process for the preparation thereof. More particularly, the invention relates to an easily openable vessel closure excellent in the seal reliability, corrosion resistance, flavor retaining property and easy openability and a process for the preparation thereof. Especially, the present invention relates to an improvement in a vessel closure comprising a can end member having a preformed opening and a flexible, easily peelable opening piece bonded to the can end member to cover the periphery of the opening.
(2) Description of the Prior Art
As an easily openable closure suitable for a package, such as a can for a drink or beverage, for example, cola, beer or juice, there has widely been used a vessel closure comprising a portion to be opened, which is defined by a weakening line (score line), a rivet formed on this portion to be opened and a pulling tab secured to the rivet, wherein if this tab is pulled, the weakening line is broken and the portion to be opened is separated.
This score line breaking type easily openable vessel closure is excellent in the combination of the seal reliability and easy openability. However, this vessel closure still has several problems to be solved. More specifically, since severe processing such as riveting or scoring should be performed in the manufacture of vessel closures of this type, the material to be used is limited to a material excellent in the processability, such as aluminum. Therefore, even if a can body is formed of tin-free steel (TFS) or tinplate, the easily openable vessel closure should inevitably be formed of aluminum. In order to prevent environmental pollution with cans and attain a resource-saving effect, it is desired that used can bodies will be recovered and utilized again. However, when the material of the can body is different from the material of the closure, it is very difficult to utilize the recovered spent can bodies again. Moreover, in a vessel closure of this type, since a separated piece provided with a tab has a sharp cut edge, the finger of a person handling the vessel closure is often hurt by this sharp cut edge. Furthermore, if separated opening pieces are scattered, a problem of environmental pollution arises.
As another easily openable vessel closure different from the score line breaking type vessel closure, there is known a vessel closure comprising a can end member having a preformed opening and an easily peelable opening piece bonded to cover the periphery of the opening. In this vessel closure, bonding of the opening piece to the can end member is accomplished by adhesion with a pressure-sensitive adhesive or heat sealing using a heat-sealable resin. A vessel closure having an opening piece bonded according to the former bonding method, however, is defective in that since re-bonding is possible by application of a pressure after the vessel closure has once been opened, it is substantially impossible to warrant a purchaser that the can has not been opened. Furthermore, in the latter bonding method, it is very difficult to keep a good balance between the easy openability and seal reliability, and especially in case of a heat seal structure having a good seal reliability, when the opening piece is peeled, a feather-like resin film is left on the opening acting as a pouring or drinking mouth. That is, a so-called feathering phenomenon takes place and an unplesant feeling is given to a drinker.
A most serious problem of the easily openable vessel closure of the type having a preformed opening is that the cut end face of the opening is exposed to the interior of the can and corrosion of this cut end face and dissolution of iron into the content take place. Various trials have been made to cover the cut end face with a resin to solve this problem.
As a typical instance of such trials, there can be mentioned a method as disclosed in the specification of U.S. Pat. No. 3,339,788, in which an inner face covering tape is bonded to the inner side of the can end member to cover the opening and the cut edge of the opening is hidden. Indeed, according to this method, since the inner face covering tape is bonded to the adhesive layer of the opening piece, the cut edge can be prevented from being exposed to the interior of the can. However, it is difficult to completely stick the inner face covering tape to the cut edge, and a space having a triangular section is inevitably formed between the inner face covering tape and the cut edge of the opening. It is believed that the reason for formation of such a space is that when the inner face covering tape is applied, it is difficult to remove air from the step portion between the cut edge of the opening and the opening piece and also when the inner face covering tape is applied, it is difficult to perform the operation of sticking the tape closely to the adhesive layer of the opening piece after the tape has stuck closely to the cut edge of the opening. If the covering tape is not closely stuck to the cut edge of the opening, corrosion, for example, rusting, is advanced on this cut edge, and a long-period seal reliability is lost. Simultaneously, dissolution of iron into the content of the can is advanced and the property of retaining the flavor of the content is degraded. Such corrosion or dissolution of iron is especially conspicuous when hot filling or heat sterilization of the content is carried out.
There has already been proposed a method in which a resin is applied in the form of a powder paint or plastisol to the step portion formed between the cut edge of the opening and the opening piece and the applied resin is heated to cover the cut edge of the opening with the resin. A resin covering formed according to this method adheres tightly to the adhesive layer of the opening piece but it is difficult to stick the resin covering closely to the cut edge of the opening which has a very small area. In fact, if a vessel closure of this type is opened, it is observed that the resin covering layer applied to the cut edge of the opening is separated from the cut edge and transferred to the opening piece, and that prominent rusting is caused on the cut edge of the opening.
As will be apparent from the foregoing description, there has not been known an easily openable vessel closure in which an inner face covering resin is tightly bonded to the cut edge of an opening preformed on a can end member and even when the vessel closure is opened and an opening piece is peeled, this covering resin is left on the can end member while adhering closely to the cut edge of the opening.